Direction indicator



Oct. 28, 1941. c. L. o'r'ro, JR

' DIRECTION INDICATOR Filed Feb. 15, 1959 5 Shecs-Sheet 1 "Oct. 28, 1941. c. L. OTTO, JR 2,260,396

DIRECTION INDICATOR Filed Feb. 15, 1959 5 Sheets-Sheet 2 ld'll fl 47 J 2 7 INVENTOR Jaw 6% BY ii N 0a. 28, 1941. c. L, OTTO, JR 2,260,396

DIRECTION INDICATOR Filed Feb. 15, 1939' 5 Sheets-Sheet 3 INVENTOR AT 0R EY Oct. 28, 1941. c. L. o'r'ro, JR

' DIRECTION INDICATOR Filed Feb. 15, 1939 5 Sheets-Sheet 4 INVENTOR (M13 fix,

ATT RN 5 Sheets-Sheet 5 Patented Oct. 28, 1941 UNITED STATES P A TENT OFFICE 2,280,396 'nmnc'non mnrca'roa Carl L. om, In. Garden city. N. 1. Application- February 15, 1939, Serial 310.256,! 16 Claims. (CI. 33-22:)

This invention relates to direction indicating instruments, particularly to liquid-stabilized instruments of the artificial datum type for airstabilizing, controlling and interpreting the indications of such instruments.

It has long been known to the art that a liquidstabilized instrument might be adapted to serve as a horizon indicator if some practical means could be found for utilizingthe full extent of the rotational isolation inherent in free liquid support, and if the isolated member could be selectively controlled by directive forces. But all means to this end so far proposed have failed to result in a successful instrument, because devices utilizing some of the potentialities have usually had limitations elsewhere in the instrument, or embody such manufacturing diillculties as to make the instrument impractical to produce. From this it follows that the means employed for attaining isolation and control, and the interrelations of these means, are critical to the success of the instrument as a whole.

It is the purpose of this invention to provide in an instrument of the. liquid-stabilized type, novel means which will work together eihciently over a wide range of performance for: (1) centering a reference member within its supporting liquid with a minimum of friction and without imposing limits on its rotational freedom. (2) Damping possible oscillations of a reference member without impairing its rotational isolation. (3) Flexibly coupling a reference member to an objective direction-sensitive member, to reduce acceleration errors. (4) Imparting inertia to a reference member in a form which will not induce errors when the instrument is subjected to temperature change. (5) Further avoiding temperature-induced errors by maintaining the temperature of the supporting liquid within a desired operating range. (6) Picturing flight conditions to the pilot in a normal arrangement. (7) Mounting the instrument away from magnetic disturbances caused by other instruments. (8) Utilizing an electric current in or near the instrument without impairing the magnetic function of the instrument. (9) Combiningall of the novel means into a single practical instrument.

As the specification proceeds, it will become apparent that instruments according to the invention may be easily and reasonably fabricated and that the principles involved will lend themselves to a variety of other uses- Further objects will become apparent, also, in the details craft. Specifically, it relates to novel means for of construction of the various embodiments shown.

In the drawings:

Fig. 1 is a schematic view in front elevation of an instrument embodying a simple form of magnetic coupling according to the invention;

Fig. 2 is a similar view of another instrument, showing a variation in the method of mounting one of the magnets;

Fig. 3 is a schematic vertical section of the instrument shown in Fig. 1, taken in the plane of the paper, with parts in elevation; I

Fig. 4 is a similar section through another instrument showing a variation of the coupling principle;

Fig. 5 is a view in vertical section taken on. the line 5-4 of Fig. 6, showing an indicator accord ing to the invention;

Fig. 6 is a view in vertical section taken on the line 6-6 of Fig. 5, with parts in elevation;

Fig. '1 is a view in vertical section with parts in elevation of an expansion chamber and temperature control switch, according to the invenbearing used in the Eon, including a diagram of electrical connecons;

Fig.- 8 is a view in front elevation of the indicia of the indicator shown in Figs. 5 and 6 as they wlriiuld appear in an aircraft in left bank and e Fig. 9 is a similar view of similar indicia adapted for optical reversal, shown after reversal, with the same attitude of the aircraft:

Fig. 10 is a view in vertical section with parts in elevation showing a modification of the invention adapted for use with course-setting indicia;

Fig. 11 is a top view of the embodiment shown in Fig. 10, showing the indicia thereof as they would appear in an aircraft in left bank and glide; I r v Fig. 12 is a detail in section of a knife-edge embodiments shown in Figs. 5, 6 and 10;

Fig. 13 is a section of an instrument mounted to produce an inverted image of the indicia thereof upon a translucent screen;

Fig. 14 is a section of an indicator according to the invention, mounted for reversal of an image of itsindicia by two concave mirrors; and

Fig. 15 is a section of an indicator according to the invention mounted for viewing a reflected image of indicia located on the side of the the same structure, but useful devices may be produced embodyingless than the whole.

Itwillbeobvioustothoseskilledintheartto which the invention appertains, that the same may be incorporated in several different constructions. The accompanying drawings, therefore, are submitted merely vas showing the preferred exemplification of the invention.

The preferred embodiments of the invention, as illustrated in the. accompanying drawings, belong to the class of liquid-stabilized instruments in which the theory of operation is based upon the rotational isolation accruing to a solid member which is neutrally buoyed in a liquid. It is to be generally understood that, unless otherwise stated,-the solid member has no inherent pendulosity, but simply I rests in the liquid as an inert body whose angular position may be controlled by properly applied external forces. Such a member may be called a refer- .ence member, asit is at once an end point of external influence and the source point of the information for which the instrument has been designed. Also, it is to be generally understood that, if the instrument is magnetically controlled,

' all the parts not concerned in-thecontrolling function will'be non-magnetic,fand further, a completeset of accessory means, such ascentralizing means for the reference member, in-

dicia, etc., is necessary for the proper functioning of the instrument.

In order to prevent the reference member from being upset by strong temporary variations of the directive forces, it is expedient to employ an intermediate direction-sensitive member, here called an objective member, which is loosely linked to the reference member in such a way that its effect thereon is greatest when the directive forces are nearly normal, and least when the directive forces most divergent from normal. This linkage arrangement is illustrated in some of its embodiments in versponse to gravity or acceleration forces; while the inner, or reference member rotates almost exclusively in response to the movements of the outer member, through the magnetic linkage.

The magnetic field of the magnet 8 will pass through the entire instrument, but it will be strongest near the magnet. In the position,

shown, its strength will vary approximately with the inverse cube of the distance from the center of the magnet. The single piece of magnetic material 9 will tend to move to the strongest part of the field, and in so doing will eventually align the reference member with the average position of the pendulum. It is intended, of course, that the mutual attraction between the magnetic members shall be very slight, which will have the effect of giving the reference member a long natural period of oscillation, and

which will also prevent undue strain upon the natural period of stability of the reference memticality indicators in the schematic drawings of Referring in detail to the drawings, l is a spherical-shell neutrally buoyed in a fluid 2 in a spherical vessel 3. A gimbal ring 4 is pivoted on the vessel on the pivots 5- around an axis passing through the center of the spherical shell I. "On the gimbal ring is journaled at H a bail-shaped pendulum l for swinging around an axis passing through the center of the spherical shell I normal to the first-mentioned axis. This 3 pendulum-constitutes the objective member of the instrument in that it is the source of gravi tational sensitivity. To link it with the reference member a strong magnet I is fixedly mounted upon it with the magnetic polar axis normally vertical in a line passing through the .center of the spherical shell I; and the reference member is likewise supplied with magnetic material which is represented, in the embodiment shown in Fig; 3 by a small piece 5 of magnetic material mounted near the periphery of the reference member I and balanced gravitationally by the non-magnetic counterweight It.

From this it can be seen that the linkage here consists of two magnets mounted upon their respective movable members for rotation about one and the same'center, being mechanically separated in their relative rotation by the intervening wall of the container, but magnet-- ically linked by the interlocking'of-the magnetic fields of the two pieces of magnetic material. The outer, orgobjective, member rotates in reber.

If both of the magnetic elements concerned are permanently magnetized, it is possible to reduce the force of'attraction on a wide swing to 1% of its normal value; but if one of the magnetic elements (in this case #9) is made of material of very low permeability, the attraction can be made to drop off even more rapidly-4t some rate greater than the inverse sixth power of the distance, depending upon the permeability curve of the material. It is not intended, however, that the force of attraction shall ever be-, come completely nil, for then the instrument might not be able to right itself after prolonged upsets, as in storage, etc. A desirable minimum value would be somewhere near the amount necessary to overcome the friction of the centralizing means. netic field may here be neglected, because of the The efi'ect of the earth's magrelatively great strength of the magnet I and weight of the pendulum 'l.

Fig. 4 represents schematically an instrument identical with that of Fig. 3, with the exception that the'magnetic element 9 and the, cormterweight III, are replaced by a permanent magnet ll held'in the center of the reference member by the support I2. The function of this instrument likewise is similar to that of Fig. 3, but the magnet ll cannot move to a stronger part of the field of the magnet 8. Instead, it aligns the reference member with the objective member by turning to align its magnetic polar axis with the local field caused by the magnet I. It is obvious that the torque applied to the reference member does not decrease with excursions of the objectivemember, but onthe contrary it increases. This instrument therefore does not have the stability of that shown in Pig. 3, but

"the theory may be used in combination with the I the arc is preferably concentric with member. The outside position is preferable, as it offers the least tendency to disturb the delicate balance of the reference member. In any event.

the reference member.

It is immaterial how-the objective member attains its pendulomty. A track or a cam surface over which the member can slide freely will do well: or the craft to which the instrument is attached might sufiice as pendulum if its average position during the stability period of.

pended reference member. The real purpose of 7 the pendulum is to compensate for prolonged alterations of the trim of the aircraft, especially during climbs and glides, and it also serves. to disconnect the directive force from the reference member during sudden starts and stops and during acrobatics. For this purpose it is, of course, not necessary for the pendulum to swing around the entire case. In practice a range of or from normal should permit compensation for changes in the trim of the aircraft, and disconnection of the instrument during sudden lateral accelerations. V

In Fig. 2 is shown schematically a variation in the method of mounting the objective member, eliminating the gimbal ring I, and thus permitting an unobstructed view of the indicia while at the same time allowing about 30 pendulum swing. Here the pendulum la is iournaled at ic-t'a directly to the case 3. The lower part of the pendulum is enlarged and formed into a hollow track, the fioor of which is indicated by the broken line II. The track forms an arc concentric with the reference member; and on the track and freely movable over the arc is a car l4, carrying the magnet I in normally vertical orientation. The car II will be recognized as constituting an objective member having the same elfective movement and carrying the magnet I in the same way as the pendulum I of Figs. 1, 3 and 4.

Also in Fig. 2 are shown indicia, of which the band it is equatorially positioned on the reference member I, and is visible through the glass window it of the case 3. It is read against the marks I1 and Ila on the outer face of the frame of the window II. The band I is always substantially parallel tothe horizon, and it reproduces truthfully the relative movements of the horizon with respect to the aircraft, although the reproduction may not appear natural to the pilot, because in looking forward he sees the aft side of the instrument, and therefore he sees the aft portion of the band It, which represents the aft portion of the horizon. A true representation of the forward portion of the horizon may be had by observing either a reflection of the forward side of the reference member, or an optical inversion of an aft image of the member. Both of these methods will be in connection with Figs. '13, 14 and 15.

Figs. 5 and 6 represent a practical flight indicator embodying the principles just discussed.

The strong objective magnet lb is here constrained to a normally horizontal position of its polar axis within the float" which'is iournalled at iU-il' in the-frame Mo to turn around a normally vertical axis. The frame llb is mounted on wheels 20-40 which run on the track lib formed in the pendulum lb which swings on a normally horizontal axis passing through the center of the reference member I, on the knife-edges 2i-2l' held in place by springs 23 and 23' and by screws 22 and 22'. On the reference member I is mounted a very small magnet 24 which is attached thereto by means of the clip 25 and the shaft 2!. This small magnet?! will followthe movements of the larger met 8b in one sense as explained in connection with Fig. 3, and in another sense as explained in connection with Fig. 4, thus orienting the reference member in azimuth as well as in verticality.-

Expressed in other terms, the two magnets 24 and") will turn with respect to each other until their respective magnetic fields are mutually stable, and then they will turn together until their resultant field is aligned with the earth's horizontal field. But the magnet lib is much stronger thanthe magnet .24-say one hundred times as strong-41o the resultant field will be essentially the same as that of the magnet to alone; and the latter magnet is constrained to a horizontal position by a relatively heavy pendulum, so the resultant field can ,be assumed to be normally horizontal. Also, the resultant field will be so much stronger than the earth's field in the vicinity of the instrument that the direct effect of the earths field upon the small magnet 24 may be neglected.

Thus, the reference member is oriented about a vertical and 'one horizontal axis by the directive action of the fields involved. It is oriented about other horizontalaxes by the attractive action of the fields, as explained in connection with Fig. 3. This completes its flexible connection with the desired outside forces through the objective member.

The reference member I of this instrument is a very thin shell which is symmetrically constructed except for the magnet 24 and its mounting, of which there is only one set. At the other pole of the reference member the same place is occupied by a pointed bearing 2'! against which the memberfioats. To provide the buoyancy, of which there is only the slightest positive excess, symmetrical hollow float chambers are mounted on the member, one of which is shown at 28. The space within the reference member contains a number of screens 29-29 arranged in parallel horizontal planes so as nearly to fill the space. The parallel arrangement is merely the preferable one, as marry other arrangements will function well, the important requirement being that the space be well covered and the mass symmetrically disposed so as not to unbalance the reference member. The remainder of the space within the member is filled with the liquid 2 which communicates freely through the ports 30 with the liquid outside of the reference member. The screens serve the double purpose of making the inertia of the inner fiuid available to the member and of damping the oscillations of the member. Whenever the member may start to rotate, each filament of the screens will have an increase of pressure on the side entering the fiuid, and a decrease on the trailing side. The summation of all these pressures amountsto a large resistance to change in motion. The fluid will eventually, come to rest relative to the member in any prolonged movement. ut the filaments should be constructedina number, size and shape which will not permit themto engage the internal fluid so strongly as'to bring it into equilibrium freedom relative to the ,iluid'and the.

The means employed for centering the reference member aretwogimbalrings. These and other parts of the reference assemblyar here shown in much heavier construction than is desirable in practice, which is done to make them more visible for description. The inner ring 3| lies in a lateral vertical plane, and the outer ring 3! lies in ahorizontal plane. Pivots are shown at 2, 21, Stand-ll. This method ofceritering is a great improvem'ent over'the methods forv merly usedin'this type'ofinstrument, inthat it permits a'full-sized spherical reference member to rotate a full revolution or more in any direction without any restrictions, and at the same time has the advantage of a very short friction radius. Another advantage of gimbal rings is that they analyze the motion of the reference member into three components corresponding to the three pivotal axes, so that separate indlcia may be used'in connection with the rings to facilitate the reading of any component separately, without interference from the others.

The rings are to be made as light as possible,

with the radial dimension flattened'and the crosssection somewhat streamlined to create a minimum disturbance in passing through the fluid. For the same reason they are both made slightly oval, each ring having its minor diameter at its inward pivoting and its major diameter at its.

outward pivoting, by which means the'various parts of the gimbal system are placed in appro-- priate portions of thejvelocity gradient of the fluid.'

Indicia are added to the reference assembly, preferably in the form of an equatorial band lib on the reference member, divided into degrees of azimuth as shown, and a. vertical wire 35 attached to the horizontal-gimbal ring 32 on an inward extension of the pivot wire 33, serving as a lubber. line against which. the azimuth graduations are read. The equatorial band lib remains parallel to the horizon, and to facilitate its in,-

ancy should be somewhat in excessof neutral, so

that an exactneutral point, or the slight buoy-,

ancy excess previously mentioned, may be attained by theaddition of small bits of heavy material to appropriate parts of the assembly in the course 9118, number of trial immersions, the heavy material being added each time in diminishing increments to the lightest side of the assembly, as shownvby the immersion. The mate-' rial added may conveniently take the form of paint with a heavy filler which can be skillfully applied to leave a smooth surface and at the same time neutralize both buoyancy and pendulositv. This method is far superior. to the ad-' justing screws, etc., provided in some designs,

which, may cause warpingof the member, orf which 'may be slightly off balance in themselves,

causing an uncertain shifting of the mass which makesa delicate balance annoylngif not imp ssible to. attain. l I I The center of volume will be naturally very close. to the center of rotation, because of the symmetry ofconstruction. If it should be necessary to adjust this relation becauseof inequalitles in construction, same can be accomplished by a. slight bending of the gimbal rings, or by adding material of the same density as the liquid V 1, as above, afterwards testing the adjustment by immersing the assembly,inlarliquidofdiflerentspecific gravity than :the'iiquid 2. However, one

of the objects of the invention .is that the instrument shall not be sensitive toslight errors in construction, and here the geometrical .symmetry, the relatively. very small volume of the assembly, and the temperature control all tend to make cen'ter-of-volume adjustment normally unnecessary.

with. temperature changes, the'liquid 2 will change in volume relative to the case 3, and the diflerence will result .in a flow of liquid through the tube 40 which leads to the expanslonreservolr shown in Fig. 7. The reservoir consists of a case 4| and a yielding member 42 which is preferabiy a Sylphon' bellows, as shown, but other yielding means, such as a piston, could be used. 1

Attached to the head 43 of the yielding member is a rod 44 for controlling, a heating'device which will be described presently. v

The purpose of the heating device is to maintain the temperature of the instrument within such limits as will avoid serious alterations of the buoyancy or fluidityof the liquid 2. For ex ample, if the instrument is tobe used at temperatures varying from +40 C. to -50 C., with actual flight temperatures rarely if ever being above 25 C., the reference member might well be balanced at 15 C., and prevented by the heating device from going below 5 C., which should permit the instrument to operate substantially within 10 C."oi its normal temperature. This degree of control should suiiice for good operation, in view of-the instruments great tolerance.

resulting from other features.

The required heat may be taken from any conveniently available source, such as hot engine oil or water, etc., but'in the absence of sucha source a device may be used such as that shown schetery "to traverse the primary circuit 41. This circuit includes an electromagnet 8 which assists the spring inholding the. contact points "-5! closeduntil the rod 44 pushes outward strongly enough to overcome both the spring. :and the magnet. In doing so'it bends the spring some what, absorbing a small amount of motion, which is released as soon as the electromagnet weakens upon separation of the contact points, permitting the spring wire 49 to straighten suddenly,

2,2oo,soo

separating the contact points il-il quickly to prevent arcing.

The primary circuit includes the primary of a conventional type medium voltage induction coil from th ring 3!. Heading is read on the horison hand against the lubber line Ii. If the pilot could look at the tail of his aircraft as related to the horizon through a rear vision mirror, he.

I! equipped with vibrator switch it and condenser 5 would see the same movements as he observes in ii, which may be shielded to avoid radio interference. An alternating current is generated in the secondary oi' the induction, coil, and is led through the wires ii-ii to a heating element oi resistance wire I! which is shown in section in Fig. i. In operation, this heating element warms the instrument until expansion of the liquid 2 reopens the switch ii. To adjust the switch ii for the desired temperature of closure, it is necessary only to adjust the amount of liquid 2 in the instrument, through the filler opening ii. The amount of iiquid in the reservoir should be small compared with the amount in the case 3. Further to increase the efilciency of the heating operation, thermal insulation 59 is applied to the outside of the case 3.

Because an electric current is always accompanied by a magnetic field, special measures must be taken to avoid upsetting the magnetic operation of the instrinnent. One such measure is to apply the resistance wire ii to the case in many narrow folds, as shown, or otherwise to bend loops of wire towards other loops to shorten the electromagnetic circuit, thus minimizing the resultant electromagnetic field in the vicinity of the mark it relative to 'the band lib. A natural, or forward-looking picture of the same sort can be obtained either by inverting an aft set of indicia such as that Just described, or by reflecting a forward set of indicia. In either of the corrected cases the result will be as shown in Fig. 9. The upper and lower hemispheres of the refer ence member may here be differently colored to represent earth and sky, so that the mark ii will move relative to the band lib and to the dark;

- of a great circle passing through the bearing 21.

In use this arrow is to' be held parallel to the grid bars il-il mounted on the grid ring 62 which is tumable and is to be set for-course on the magnet lb. This self-neutralizing effect can be enhanced by increasing in number the loops in the wire i1, which in turn becomes possible when the voltage of the secondary of the induc-' tion coil is increased. The residual field reaching as far as the magnet 8b is thereby rendered so weak as not to endanger the permanent mag-s netism of the magnets ib or II, which magnets should have a high coercive force for this reason.

The second meuure is to construct the vibrat-' ing switch ii of the induction coil to vibrate at a frequency high enough so that no appreciable movement of the magnet lb can occur before the current in the secondary is reversed. Actually this does not require a very high frequency. Since the electric current moving in each direction is the same, there can be no residual effect on the permanent system.

As a third measure, the reservoir, induction .coil and all of the direct current circuit should be remote from the remainder of the instrument, being connected only by the wires 55-55 and the tube 40.

It is intended that a suitable magnetic compensator be used with this instrument, but it need not here be shown, because compensators are well known to the art. It may be said, however, that the compensator should be of such proportions that it will correct the instrument from a distance above it equal to or greater than the diameter of the instrument, or from a greater distance below, which positions will permit substantially even compensation over the entire range of the penduious movement of the magnet ib.

Fig. 8 is a view in front elevation of the window 31, showing th indicia as they would appear in left bank and glide with a heading of due north magnetic. The mark 36 is fixed on the window 31 and represents the aircraft. It is read for bank against the ring 32, which tilts fore-and-aft with the aircraft, but laterally remains parallel to the horizon. Pitch is read by observing the separation of the'horizon band lib the index it. I V

In addition, a cross 64 is mounted on the ring ll over the bearing 21, and a similar cross ii is mounted below the center of the window 31c. In a properly installed instrument, these two crosses will be superposed during strictly level fiight, but changes in pitch or bank will separate them. The movements of th cross 65 relative to the cross 64 are exactly the same as those of the stabilizer control lever relative to the air-' craft. Th crosses do not appear to rotate, but merely drift apart with changes in tilt of the aircraft. Thus, to normalize the aircraft in pitch and bank after any ordinary maneuver, it is necessary only to move the control lever in the direction indicated by an imaginary pulling of the cross 65 back over the cross 64.

This modification is intended for lower cost production and less severe flight conditions than the instrument shown in Figs. 5-7, and therefore has no temperature controlmechanism. Expansion is taken care of by the bellows 66, the head of which is provided with a traction loopv 61 which enables the liquid 2 to be de-aerated and then to be kept under pressure to avoid bubbles. The magnet is mounted on pivots l9--l9' on a gimbal assembly similar to that shown in Figs.

l, 3 and 4. The glmbal assembly is hung on knife-edges 2lc formed on the links 68 which join the lower cowl shell 69 with the upper cowl shell 10, which is fastened to the frame ll.

Fig. 12 is a detail of the knife-edge bearing ,construction used in the pendulum mounting, showing the knife-edge 2| in position in a notch in the button 12 which has pendulum lc.

Figs. 13, 14 and 15 show three species of means for obtaining a forward horizon eifect in an instrument of this type, as discussed in connection with Figs. 2, 8 and 9. Figs. 13 and 14 illustrate two means for inverting an aft image, while Fig. 15 shows a means for reflecting a forward upright image. In each case the indicia will obviously have to be arranged to fit the optical system employed, in order that the figures, etc. may stand correctly in the final view.

Fig. 13 shows a housing containing an indicator unit 3. a lens l4 and a translucent screen Ii.

been pressed into the An electric lamp II and reflectors l'landlt serve to illuminate. the indicia. The lens projects" a real, inverted' image of the indicla onto the screen- This method of viewing the'indicia'is intended for night use. In the daytime the screen ll may be removed if it is logged by day light, and the inverted image viewed directly,

providedthe lens islarge enough to .give av a dfleldofview. l4. shows'a method; of projecting anIinverted image suitable for daylightuseiaswellas for night use if illuminated. This is a. top' view inverted image 8!, as shown- ,A 'andB repree sent two source points. Llght'from these-points;

striking the mirror 19 .convergesat' A and B, re:--

spectively. The mirror 80 has 'sucha broad sure 1 case having a spherical chamber containing element, two oval rings encircling the element,

and encircling the inner and, pivoted on its axisto the inner ring on the maior axisthereof and pivoted-on its major axis to the case on a a normally horizontal axis thereof, and a liquid 10 substantially filling the space within tbeehamber and surrounding and hydrostatically supporting.

relation.

2. :A devicefproviding. a rotationally dampeddirection-sensitive elementwithin and rotation- "ally isolated from'a. case about all axes with re- 'spect thereto, including a sphericalchamber form edwithin said case, a liquid substantially.

face that. theimage. 8! may ,be viewed without" filling said chamber, said rotationallyisolated ditheinterposition of a screen. However, with sufflcient illumination offthe indicia. .a translucent screen may be placed atll along the line'A'-'- In thisconnectiomi a screen has the great advantage of eliminatingall parallax, as well as ren.

' .dering the image visible from a much wider an-.

. forward the window 31 may be exposed to much stronger light from the outside than it woul have if located on the instrument panel.

All three of the arrangements shown in Figs. 13, 14 and 15 result in removing thesensitive part of theinstrument from the place where the image is observed, which place is usually crowded with otherinstruments whose operation might interfere magnetically with any accurate direction indication having its source in that spot.

The removal of the sensitive elements may be accentuated, as for example in Fig. 15 by removing the case 3 to the left away from the mirror 82,

, and compensating optically by making the mirror 82 larger and slightly concave. IBy this means the instrument proper might be mounted in back of and slightly above the pilot; where it would be well isolated and have good illumination from above.

The arrangement shown in Fig. 14 is capable of even more variation, in which the instrument may be above, below, or at either side of. the point of observation, wherever there is mos room.

All of the modifications of the invention here discussed have the further advantage that they.

combine in' one field, and in some casesin one moving index, all the information necessary in blind flight for stabilizing an aircraft and keeping it in its. compass course. I

It will be appreciated that various changes and modifications may be made in the device as shown in the drawings, and that the same is submitted in an illustrative and not in a limiting sense, the scope of the invention being defined in the following claims.- I claim as my invention: 1. A device providing an element having unireetion-sensitive element being disposed substantiallyin equipoise withinand about a portion or said liquid,"said element having a substantially spherical external shape andhavingliquid-mashing projections on its interior, and a gimbal system constraining said element translatlonally to a central position within said chamber. 3. A magnetic averaging device including a case having a spherical chamber, a neutrally balgo'anced rotatable reference member centrally .lo-

cated within said chamber, magnetic material mounted on said member in' a fixed eccentric position relative thereto, a liquid within said cham-,

her and surrounding and hydrostatically suPPOrt-. ing said member to render it fundamentally inert to acceleration forces, and a magnetic objective member rotatably mounted outside of said chamberabout substantially the same. center of rotation as said reference member for controlling 40 magnetically the average angular position of said reference member by attracting the eccentric magnetic material thereof toward the objective member.

4. A stabilized magnetic direction indicator ineluding a primary strongly magnetic element rotatably mounted around a normally vertical axis to align itself with the horizontal component of the earths magnetic field, a secondary weakly magnetic element mounted for rotation around a normally vertical axis to orient itself in azimuth by tending to assume a definite position in'the magnetic field of said primary element, a liquid tight housing surrounding said weakly magnetic element, and a damping liquid in said housing within which said weakly magnetic element is immersed and by which its movements are damped.

5. A magnetically stabilized combination direction indicator, including a support member, a reference member neutrally mounted thereon for no universal rotation relative thereto around a given center, an objective member mounted on said support member for pendulous rotation relative thereto around said center, a first magnet pivotally mounted on said objective member for rota- 05 tion around a normally vertical axis as a compass needle, and a second magnet mounted on said reference member in a fixed eccentric position relative thereto for orienting the reference member both in azimuth and inclination by tending to align itself with and approach the first magnet.

6. A magnetically stabilized direction indicator, having a support member, a magnetically responsive' reference member neutrally mounted thereon vfor universal rotation relative thereto versal free rotation, including in combination a 7 around a given point as a center, a pendulum the; inner .said ring being pivoted on its said elementlall said axes being mutually per-l pendicularwhen element and case are in normal pivoted on said support member around a normally horizontal axis passing through said point and having a track formed thereon in the form of an arc of a circle, having said point as a center, in a plane including said axis and the center of gravity of said pendulum, and a car supported on said track for movement therealong and bearing a magnet for movement therewith along said arc.

7. A thermally stabilized device providing a rotationally isolated element, including a case having a spherical chamber, said element being substantially spherical, rotationally free and centrally located within said chamber, a liquid substantially filling the space within said chamber and surrounding and hydrostatically supporting said element, a heating element in thermal contact with said liquid and a temperature responsive element responsive to the temperature of said A liquid and connected to control said heating element for stabilizing the supporting and dampin properties of said liquid.

8. A device providing a thermally stabilized hydrostatic support for a sensitive element, including a case forming a closed chamber, said element being located within said chamber, a liquid substantially filling the space within said chamber and neutrally supporting said element, a heating element in thermal contact with said liquid, and a pressure-sensitive yielding member in contact with said liquid and connected to control said heating element in response to pressure changes of said liquid.

9. A thermally stabilized direction indicator including a case having a chamber therein for holding a liquid, a rotatably free magnetic direction-sensitive element within said chamber having a slow period of oscillation, a liquid within said chamber surrounding and hydrostatically supporting said element, an electric resistance wire in thermal contact with said liquid, and an electric induction coil equipped with an interrupter and operably connected with said wire and with a source of electric current for supp ssaid wire with an alternating current of higher frequency than said period of oscillation, for heating said liquid to render stable its hydrostatic support of said element.

10. A flight indicator for aircraft, including, in combination, a closed thermally insulated vessel having a spherical interior portion, a liquid substantially filling said vessel, a flexible member connected with said vessel responsive to volume changes of said liquid, an electric heating element screen within said shell, a gimbal system for constraining said shell to rotation around a central point within said spherical portion of said vessel and shell, a semi-circular pendulum embracing said spherical portion of said vessel and pivoted for rotation around a normally horizontal axis passing through said central point, a track formed on the major arc of said pendulum, a car freely movable over said track, a magnet mounted on said car for rotation around a normally vertical axis havlngits field normally linking with that of the shell magnet for stabilizing the shell, indlciafor observingtheangularpositionofsaid shell relative to said vessel, and optical means for arrangin an image of said indicla into a normal fl l 9 11. In a direction indicator, in combination, a spherical case, a liquid therein, a spherical sensitive element hydrostatically supported within said liquid, and articulate means for centralizing the element with respect to the case, includin a plurality of gimbal members serially pivoted to the case, to each other and to the sensitive element, respectively, about at least three mutually normal separate axes passing through the common center of the case and element, whereby the sensitive element is left free to rotate in any dirction independently with respect to the case said members being so thin as to cause a minimum of fluid friction between said element and liquid upon relative motion thereof while serving to centralize said element with respect to said case.

12'. In a direction indicator, the combination with a liquid-immersed rotationally isolated direction-sensitive element having its centers of buoyancy, mass and rotation substantially coinciding, of a substantially spherical chamber within said element provided with projections extending inwardly from said element, and a port in said element permittingthe liquid in which the element is immersed to enter said chamber and to mesh with said projections for damping rotational movements of the element, said element being constructed to contain more than its own weight of effective damping liquid.

13. In a direction indicator, the combination with a hollow rotatable element including a magnetically responsive member, of supportmeans therefor permitting the revolution thereof in any direction, a damping liquid within the hollow of said element and supplying the major part of the inertia thereof, and magnetic field producing means mounted on said support means close to the element for controlling the average angular position of aid element through magnetic linkage with the magnetically responsive member of said element.

14. A direction indicator a direction-sensitive element, means for operably supporting said element including a liquid furnishing hydrostatic netic field, said shell upon drift from such alinement, responding to realinement at the said natsupport thereto, means for altering the temperature of said liquid, and means responsive to volume changes of said liquid for stabilizing the supporting property. thereof by controlling the temperature altering means. t

15. In a direction indicator, the combination with a spherical container having therein a body of liquid, of a hollow spherical direction indicating shell therein whose interior'communicates period of oscillation longer than one minute and containing a portion of said liquid of greater weight than its own weight for damping purposes, a plurality of screens elements rigidly held within said shell for frictionally coupling it with the en'- closed liquid, and magnet means rigid with the shell tending to aline it with an external maguralperiod of oscillation.

18. A direction averaging device comprising a container having therein a liquid, a perforated spherical shell immersed in the liquid, a screen element secured within the shell, and magnetic responsive means carried by the shell by which a selected axis of the shell is urged to parallel aline ment with the direction of action of an external magnetic field.

OARLLO'I'I'QJI. 

